I currently use CodeIgniter for an MVC framework in PHP because it's a very bare framework, is there anything even more bare than CodeIgniter?
Is there a reason you are looking for anything lighter? I personally found code-igniter to be too low-key and it simply doesn't do enough for me - which is the purpose of a framework anywany. Also, I didn't find its code quality to be in the same class of other profilic frameworks such as Zend's and Symfony.
There's always Rasmus Lerdorf's "No-Framework MVC Framework". That's about as "light" as you can get.
You probably want to roll your own solution. I've written a small set of php pages that uses MVC (kinda) as controller/view scripts, a set of functions for data retrieval and a central index.php script to map urls to the proper files.
Of course, a good question is why do you need anything lighter? If you are having performance issues, I doubt a lighter framework is the solution.
From what I can tell, CodeIgniter is about as light as they get for PHP frameworks. I've tried it out a little and found it to be reasonably fast compared to CakePHP.
From reddit links a few months back: PHP framework comparison benchmarks. With caching, CodeIgniter handled about 1/3 fewer requests/second than bare PHP vs. 1/10 with Zend and 1/50 for CakePHP. Depending on traffic levels you're expecting though, any of these may work just fine, and easier development may be worth the performance hit CakePHP comes with.
Based on your question, I'd say you should try to create an MVC patterned app using just PHP/MySQL.
I think you'd gain a lot of valuable insight, and it would probably generate much more specific questions.
I've been using LightVC a lot lately and it makes CodeIgniter look like a heavyweight. Beware that it's just a view-controller framework. If you want some kind of data access support, you'll need your own ORM layer. I've been using CoughPHP for that. It's written by the same guy and fairly lightweight as well.
Now keep in mind, LightVC and CoughPHP give you MVC only. No helpers, none of that. Just basic MVC organization and routing. In my case that's exactly what I wanted. From what you've described it sounds like it's exactly what you want as well.
You could use something like TinyMVC or the already mentioned LightVC or you could use an ORM such as Doctrine for the Model and a template system like Smarty or Dwoo for the Views and for the Controller you could use Zend_Controller or Horde Routes
You could also try using phpDataMapper as your Model layer. The entire codebase is under 100k, uncompressed.
How about my tweetable "framework":
$g=$_GET;$c=@$g['c']?$g['c']:'Home';
if(!@include"c/$c.php")die('fail');
$m=method_exists($c,$m=@$g['m'])?$m:'index';
$o=new$c;$o->$m($g);
I have been working on a MVC PHP web development framework and it is initially very light. You can add extensions to the system in a latter stadia if you need to.
You can check it out it's called the RDS Framework
It is open source and there is a nice documentation and some video tutorials to get you started.
It seems like everyone and their mother is building a PHP framework these days. I too have hopped on the bandwagon with my own framework: Sonic.
The Core framework clocks in just under 1000 lines of code (~21kb) and handles routing to controllers and actions (views), using layouts, rendering views within views, etc.
You can also set an App Delegate class to receive events as the application runs, and it has a native implementation of Facebook's Big Pipe.
Beyond the core stuff there is also an extremely light ORM as well as database and cache layers.